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'98 Honda BF50AW - VERY LOW idle at trolling RPMs

S

SoCalAngler

Regular Contributor
Took my Arima 15 boat out fishing last week. Motor ran great! Trolled at approx. 2mph for approx. 4 hours, which for my outboard, was at idle (IN gear). I noticed several times that the motor ran at approx. 500-600 rpm instead of the usual idle (not in gear) rpm of 900-1000. When at this idle it would run a little rough, almost wanting to stall. This is the first time, since rebuilding carbs several months ago, that I have put this many trolling hours on the motor. When not at 500-600rpm trolling, it would run at approx. 700-800 rpm which was better. It seems the lower RPMs were AFTER the first 1-2 hours of trolling. NOTE: IN gear, and general running, it runs great. Again, at idle (NOT in gear), it idles at 900-1000rpm.

A couple of questions…..


  1. What can be causing the motor to idle great (900-1000rpm) at idle (NOT in gear) and then drop to as low as 500-600rpm when in gear (although not always, but several times over 3-4 hours)?



  1. Considering a Suzuki 2.5 long shaft as a kicker for trolling. It is 31# and water cooled, as opposed to Honda 2.3 long shaft (31#) which is air cooled and rather noisy. It would also serve as an emergency back up motor in the event my trusty Honda 50 let me down. Thoughts on this trolling motor to avoid using my 50hp for extensive trolling hours?


Thanks!

 
These things SHOULD idle all day without trouble. That's what these 4 strokes do! Clearly your carbs are clean, so I'm wondering if you synchronized the carbs after cleaning. Many times you can get away without it, but when it comes to running like you are asking for, carb sync is always part of a good plan. All 3 carbs should support the engine equally. That's important because if one of the carbs isn't pulling the same load as the rest, it may stop pulling enough fuel (due to low vacuum) and drop off line on you - causing the motor to continue running a little rougher on just 2 cylinders at a lower rpm..... That's where I would start here..... If you don't have a bank of gauges for this purpose, they aren't expensive, and they're worth every dime they cost. You don't need anything fancy. 40 bucks will get the job done! Holler if you need a hand.... There's quite a bit to it if you've never done this before.-Al https://www.amazon.com/s?k=carb+syn...nc,automotive,761&ref=nb_sb_ss_c_2_9_ts-doa-p
 
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No argument with what Alan says. It shouldn't be changing idle like that.
But, having said that, your question refers to if you should be using it to troll like this or not and should you switch to a "kicker".
My opinion is no and yes.
Most engine's do their work in the most efficient manner above idle in the "power band" that they were designed for. Too much idling will keep combustion temperature low and that will contribute to carbon build-up on piston crowns and valve faces and stems. Too much of that and engine efficiency will suffer. Just ask any cop car mechanic what hours of idling does to those engines. I would not use a Honda 50 to troll unless it's on a 40' +.fishing boat that would require the engine to be "worked" as it was intended to.

I say yes to finding a suitable one cylinder to take over the trolling duties. I have no preference for either of the engines you listed but I believe either one would be a better choice for extended trolling in lieu of your sweet 50.

Just my opinion.

Good luck.
 
If somebody wants to run a kicker, I'm fine with that. I would just note that a TON of these 40-50hp Honda's are used on pontoon boats that only rarely see anything over an idle. In my experience, they work fine like that, and many have been doing so for years! It's one of the reasons they're so popular for this application! If you want to see how to screw one of these up with extended idling, just remove the thermostat. Nothing will happen right away, but after a season of abuse like this, you may notice the compression dropping as carbon starts building on the rings. If that does happen, many times they can be brought back with the installation of a new thermostat, a new water pump if it's over 3-4 years old (or you don't know how old it is for sure!), change it over to synthetic oil, then go run the hell out of it! That's me though. Do as you wish. -Al
 
Thank you both Alan and jgmo. Always appreciate the assistance!!!

Yes, I do have synchronizer gauges and did synchronize the carbs right after the rebuild. I can check them again to see if there has been any change. The boat sees very little use (due to work and other obligations)....less than 10 hours since carbs rebuilt . However......I just retired in July and now the boat will see much more use!!!

What should idle be once placed into gear?
At idle (NOT in gear) I set it at 900-1000 rpm; it seemed to be the sweet spot and was close to OEM specs.

The thermostat is approx. 2 years old but has had very little use (less than 20 hrs.) and the pee stream is very strong. I do plan on a full service (engine oil/filter, lower end oil, thermostat, water pump impeller, and housing) this Spring.

Beyond getting my 50 tuned to spot-on, I will probably go with a 2.5hp kicker..... both for safety/having a back up, but also for the planned hours and hours of trolling.

Thanks!
 
I generally set them where they work best. Too low, and they can be a pain in the butt until they warm up, which I find irritating. Too high and they can really bang when shifted into gear, especially with a stainless prop. 900-1000 is fine....
 
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